Recently, the electronic technology has made a remarkable progress and various devices have been reduced in size and weight. In accordance with the miniaturization and the weight reduction of electronic devices, batteries that supply power to such devices are also required to be smaller and lighter. Non-aqueous lithium secondary batteries (hereinafter, also referred to as lithium ion secondary batteries), which have a small volume and are lightweight, can supply large amounts of energy and are used as power sources mainly in small household electronic devices such as cellular phones, personal computers and video camcorders.
An electrode of a lithium ion secondary battery is generally prepared by, for example, kneading a powdery active substance together with a binder, a liquid substance and optionally additives such as a conductive auxiliary to give an electrode-forming composition (hereinafter, also referred to as the “slurry”), applying the slurry onto a metallic collector such as aluminum, copper, nickel, titanium or stainless steel, and drying the slurry to remove the liquid substance, thus resulting in an electrode in which a layer containing the active substance is formed on the collector.
The binder contained in the slurry is required to effectively bond the active substance to the collector as well as to stabilize the slurry. A stable slurry is such that an active substance is dispersed homogenously and the slurry can maintain a constant viscosity for at least several days without precipitation or separation of the active substance or any other components in the slurry and can be applied to a collector uniformly so as to form a smooth layer on the collector which contains the active substance in a uniform composition.
It is known that a vinylidene fluoride resin be used as a binder in order to obtain a stable slurry.
There have recently been accidents caused by the ignition of lithium ion secondary batteries and associated recalls of products. Since secondary batteries are mounted on some types of automobiles such as electric cars and hybrid cars, the safety and the reliability of lithium ion secondary batteries have become of greater importance than ever.
Extreme overheating and ignition of secondary batteries such as lithium ion secondary batteries are frequently attributed to short circuits (shorts) that take place within the batteries (hereinafter, also referred to as internal short circuits). A conductive contaminant in a secondary battery can cause electrodes to be physically or chemically short circuited. Thus, a binder solution used for the formation of electrodes is required to be free of any conductive foreign substances such as metallic foreign substances.
A large number of techniques to filter a polymer solution have been heretofore known (for example, see JP-A-2004-105865).
JP A 2004-105865 discloses that a polymer solution is filtered through a plurality of filtering steps whereby the filtration life of filter membranes is extended and the filtration accuracy is improved.
Although many techniques for filtering a polymer solution are known, the required filtration accuracy differs in accordance with the use application of the filtered polymer solution. It is not realistic to remove all minute foreign substances in the solution without considering the filtration accuracy because the productivity of the filtered polymer solution is deteriorated and the costs are increased. Filtration accuracy which enables suppressing of the occurrence of internal short circuits (shorts) in non-aqueous batteries such as lithium ion secondary batteries has not been considered.
The present invention has been made in view of the problems in the art described above. It is therefore an object of the invention to provide a process for producing non-aqueous battery electrode-forming binder solutions which includes a step of performing filtration with a filtration accuracy that enables suppressing of the occurrence of internal short circuits (shorts) in non-aqueous batteries such as lithium ion secondary batteries, and to provide a non-aqueous battery electrode-forming binder solution that is obtained by the process.